Objective: To quantify the incidence and degree of endotracheal tube intraluminal obstruction after mechanical ventilation and its relation to time of intubation. Design: Prospective observational study. Setting: A 14-bed medical-surgical intensive care unit at a university-affiliated teaching hospital. Patients: Ninety-four endotracheal tubes used in 80 patients requiring mechanical ventilation for more than 12 h. Interventions and results: Acoustic reflectometry was performed in every endotracheal tube after patient extubation to measure its volume reduction. The intraluminal volumes of used endotracheal tubes in mechanically ventilated patients were significantly lower than those of unused tubes of the same size (5.52±0.92 ml3 versus 6.54±0.79 ml3, p<0.05). The mean difference in endotracheal tube segment volumes was 15.2% (range 0-66%). Volume reduction was above 10% in 60.8% of the tubes. In 22% of endotracheal tubes the remaining inner diameter was less than 7 mm. Reduction below this figure was less frequent (9.3%) in tubes 8 mm or more (p<0.05). The percentage of endotracheal tube volume reduction was not associated with the duration of intubation (r=-0.09, p= n.s.) Peak pressure measured before extubation did not predict obstruction (r=0.11, p= n.s.) Conclusions: Inadvertent endotracheal tube obstruction was common in patients requiring mechanical ventilation and may be significant as early as at 24 h. Moderate obstruction in endotracheal tube lumens should be suspected in cases of difficulties in weaning, even in patients who were ventilated for less than 1 day.